Here's a very rough guide on how to add a new voice to Pillars of Eternity 2, without replacing an existing one. From the code side of things it's not too complicated, you'll just need 2 blocks of code. The main complication lies in creating the audio files and naming them correctly so that the game will know what to do with them. Go to the PillarsOfEternityII_Data directory, you can find this by default: Steam: C:\Program Files (x86)\Steam\steamapps\common\Pillars of Eternity II Deadfire